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'[OT](Zero crossing)'
1998\05\11@130651 by Mike Keitz

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On Mon, 11 May 1998 11:43:56 +0200 Caisson <spam_OUTcaissonTakeThisOuTspamTELEBYTE.NL> writes:
>> Van: FScalini <.....FScaliniKILLspamspam@spam@AOL.COM>

>> I've seen zero crossing detection of AC currents come up frequently
>on
>this
>> list lately, especially with respect to dimming applications.  What
>is
>the
>> practicality of zero crossing detection?

>The AC current is mostly switched by a Triac.  This device will, when
>switched, stay that way until there is no voltage left over the
>Kathode &
>Anode. then it switches off and waits for a new (trigger) signal (on
>the
>Gate).

Triacs and SCRs belong to a class of semiconductors called "thyristors"
The structure of these devices contains internal feedback so once
switched on, they will switch to full saturation and stay on even if the
gate input signal is removed.  Only stopping the current flow through the
device (for most devices, for a rather "long time" of several hundred us)
will allow it to turn off.

Aside from the big disadvantage of needing some factor in the output
circuit to turn them off, thyristors have lots of advantages.  Only a
small turn-on drive signal is required, and only for a short time until
the device turns on.  Since the internal feedback will always turn it on
fully, it is hard to create circumstances where the thyristor burns out
from not being turned on completely.  Devices with large current and
voltage capacities are inexpensive to make.

In an AC circuit, the voltage and current go to zero twice in each cycle
(though not necessarily at the same time).  Thus a thyristor switch will
be able to turn itself off at certain times.  Often the object is to make
a thyristor conduct for only part of the time to reduce the power
delivered to the load (such as a lamp).  The driving circuit can only
control the turn-on time.  The turn-off time is determined by the phase
of the power supply voltage.  So the driving circuit needs to synchronize
itself to the power supply in order to turn the thyristor on at the
proper times so the on-time is as desired.  A "zero crossing detector"
function is a necessary part of the driving circuit to achieve this
synchronization.

> If you switch your current when it is Zero, it will go up
>(or
>down) gradualy (along the sine-wave) and thus _not_ generate to much
>problems with coils and the like.

Now we move to an advanced topic.  Switching a transformer is not that
simple.  You do want to switch off at a zero current point.  A triac will
do that inherently.  However, switching on at the zero voltage point is
not optimal.

The reason is that commercial transformers are built with as little iron
as possible.  The core comes very close to saturation during each
half-cycle during normal operation.  During continuous operation, at the
zero voltage point there is still some "magnetic charge" (somewhat
related to current) in the core in the opposite direction.  Starting the
half-cycle with this negative magnetism prevents the core from
saturating.

However, at switch-on the core is not magnetized.  The first half-cycle
will saturate it.  This causes a large audible "thump" from the
transformer and a big surge of current from the line (and through the
triac).  The effect can be noticed using a manual switch.  The time when
a manual switch closes is random relative to the power line.  Sometimes
the transformer will "thump" and the lights dim, sometimes not.  Using a
solid-state relay with zero-voltage turn-on, the effect will happen every
time.

I talked to the relay manufacturer about this.  They said that
transformers were problematic.  The optimum turn-on time is about 1/4 to
1/2 through a half-cycle, but it varies.  They didn't recommend switching
large transformers with SSR's, unless the current surge is acceptable and
a much higher rated SSR used to prevent damage.




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1998\05\11@161612 by Sean Breheny

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At 12:53 PM 5/11/98 -0400, you wrote:
>On Mon, 11 May 1998 11:43:56 +0200 Caisson <caissonspamKILLspamTELEBYTE.NL> writes:
>>> Van: FScalini <.....FScaliniKILLspamspam.....AOL.COM>
>
>>> I've seen zero crossing detection of AC currents come up frequently
>>on
>>this
>>> list lately, especially with respect to dimming applications.  What
>>is
>>the
>>> practicality of zero crossing detection?
>
>>The AC current is mostly switched by a Triac.  This device will, when
>>switched, stay that way until there is no voltage left over the
>>Kathode &
>>Anode. then it switches off and waits for a new (trigger) signal (on
>>the
>>Gate).
>

<SNIP>

{Quote hidden}

<SNIP>

Speaking of triac based light dimmers, when designing one of these, does
one need to take into accout the heating and cooling of the lightbulbs?
When the triac switches off, the lightbulb will cool somewhat,and its
conductivity will be higher when it gets switched on, which I would
immagine would cause LOTS of current spikes at low duty cycles. Is this
ONLY an EMI concern, or might it decrease the lifetime of the bulb, as well?

Sean

+--------------------------------+
| Sean Breheny                   |
| Amateur Radio Callsign: KA3YXM |
| Electrical Engineering Student |
+--------------------------------+
Save lives, please look at
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Personal page: http://www.people.cornell.edu/pages/shb7
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Phone(USA): (607) 253-0315

1998\05\12@000702 by Chris Eddy

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face
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Sean Breheny wrote:

> <SNIP>
>
> Speaking of triac based light dimmers, when designing one of these, does
> one need to take into accout the heating and cooling of the lightbulbs?
> When the triac switches off, the lightbulb will cool somewhat,and its
> conductivity will be higher when it gets switched on, which I would
> immagine would cause LOTS of current spikes at low duty cycles. Is this
> ONLY an EMI concern, or might it decrease the lifetime of the bulb, as well?
>

In the work that I have done with lamps and dimmers, I found that when the bulb
filament is relatively 'hot' between on cycles it prevents the bulb from
experiencing inrush.  If I recall my work some years back on lamps, there is a
saw tooth of current generated, and the longer the period (and thus off time)
the higher the saw tooth of current max goes.  BTW, the one thing I looked into
was whether or not all of the pulsing DC caused a loss of life for the light
bulb.  I really poked around some of the manufacturers, and none that I talked
to had ever done any data on PWM effects on hours of life.  Yet it is a common
practice. ( or is that practise, spelling fanatics?)

Chris Eddy, PE
Pioneer Microsystems, Inc.

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1998\05\12@140114 by Harold Hallikainen

picon face
On Mon, 11 May 1998 23:59:01 -0400 Chris Eddy <ceddyspamspam_OUTNB.NET> writes:

>  BTW, the one thing I
>looked into
>was whether or not all of the pulsing DC caused a loss of life for the
>light
>bulb.  I really poked around some of the manufacturers, and none that
>I talked
>to had ever done any data on PWM effects on hours of life.  Yet it is
>a common
>practice. ( or is that practise, spelling fanatics?)


       I think it's "practice" (at least here in the US).  I do think
there is shorter lamp life if a filament is indeed run on DC, as there is
uneven vaporization of the filament.  However, since we are actually
driving the lamp with AC (though not a sine wave), I think the filament
vaporization is even with phase controlled lamps.
       Further, it seems that the "steep edge" of the phase controlled
waveform will cause more stress on a filament than the smoother sine wave
(as evidenced by lamp "singing"), that steep edge is present when the
lamp is operating at reduced temperature.  I think any loss of lamp life
due to the steep edge is more than made up for by improved lamp life due
to the lower temperature.


Harold


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1998\05\13@064949 by Martin McCormick

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       I used to hear about something called the halogen cycle when I
worked as a technician with audio visual equipment.  The idea was that
a halogen lamp filament gives off vapor when run at reduced brightness
and that the vapor redeposits back on to the filament when it was run
at full brightness.  I don't know if this is for real or one of those
bits of urban wisdom that turn out to be either just flat wrong or based on
out-dated information.

       The halogen bulbs used in projection equipment are usually quite
expensive for light bulbs so people try to baby them as much as possible
when the replacement bulbs come out of their pockets.

Martin McCormick 405 744-7572   Stillwater, OK
OSU Center for Computing and Information services Data Communications Group

1998\05\13@114302 by Andy Kunz

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face
At 05:46 AM 5/13/98 -0500, you wrote:
>        I used to hear about something called the halogen cycle when I
>worked as a technician with audio visual equipment.  The idea was that
>a halogen lamp filament gives off vapor when run at reduced brightness
>and that the vapor redeposits back on to the filament when it was run
>at full brightness.  I don't know if this is for real or one of those

All light bulbs do this to some degree.  The difference is that halogen
bulbs CYCLE the deposits back onto the filament.

Andy


==================================================================
                    Andy Kunz - Montana Design
         Go fast, turn right, and keep the wet side down!
==================================================================

1998\05\13@124826 by Mike Keitz

picon face
On Wed, 13 May 1998 05:46:34 -0500 Martin McCormick
<@spam@martinKILLspamspamDC.CIS.OKSTATE.EDU> writes:
>        I used to hear about something called the halogen cycle when I
>worked as a technician with audio visual equipment.  The idea was that
>a halogen lamp filament gives off vapor when run at reduced brightness
>and that the vapor redeposits back on to the filament when it was run
>at full brightness.  I don't know if this is for real or one of those
>bits of urban wisdom that turn out to be either just flat wrong or
>based on
>out-dated information.

No, it's approximately true.  Running a halogen bulb at 10-20% less than
rated voltage will shorten it's life.  The gas in the bulb is a mixture
(including halogens) at high pressure.  When the temperature is high
enough, the "halogen cycle" returns most of the tungsten that comes off
the filament back to the filament.  The filament is the same as in a
regular bulb, but due to the halogen cycle it can be run hotter, thus
these bulbs have a brighter, whiter light.

At much less than rated voltage, either type of bulb will last a very
long time because the filament doesn't get hot enough to "evaporate" in
the first place.


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'[OT](Zero crossing)'
1998\07\05@180642 by Andrew Russell Morris
picon face
At 03:51 PM 5/11/98 -0400, you wrote:
>At 12:53 PM 5/11/98 -0400, you wrote:
>>On Mon, 11 May 1998 11:43:56 +0200 Caisson <KILLspamcaissonKILLspamspamTELEBYTE.NL> writes:
>>>> Van: FScalini <RemoveMEFScaliniTakeThisOuTspamAOL.COM>
>>
>>>> I've seen zero crossing detection of AC currents come up frequently
>>>on
>>>this
>>>> list lately, especially with respect to dimming applications.  What
>>>is
>>>the
>>>> practicality of zero crossing detection?
>>
>>>The AC current is mostly switched by a Triac.  This device will, when
>>>switched, stay that way until there is no voltage left over the
>>>Kathode &
>>>Anode. then it switches off and waits for a new (trigger) signal (on
>>>the
>>>Gate).
Note that a triac or SCR does not turn off until the current through it
goes below the holding current, not necessarily when the voltage across it
is zero.

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